"tan .delta." is available as in index of heat generation when a vulcanized rubber composition having reinforcing particles incorporated, is subjected to repeated deformation. For example, it is known that tan .delta. at from 50 to 70.degree. C. may be made small in order to reduce the rolling resistance of a tire tread and to reduce the fuel cost. On the other hand, the damping properties on a wet road surface, etc., are better, when tan .delta. in the vicinity of 0.degree. C. is larger. As a method for improving this temperature dependency of tan .delta. i.e. for reducing tan .delta. at from 50 to 70.degree. C. and increasing tan .delta. in the vicinity of 0.degree. C., it has been tried to incorporate silica as a filler. For example, JP-A-3-252433 discloses that silica and a silane coupling agent are incorporated to SBR prepared by solution polymerization. Further, JP-A-7-165991 discloses that as a silane coupling agent, a bifunctional one which is reactive with silica and rubber, is particularly excellent.
In this connection, it is known that when particles having silanol groups on the surface, such as silica particles, are kneaded into a diene type rubber together with a silane coupling agent such as bis(3-triethoxysilylpropyl)-tetrasulfone, followed by vulcanization, the particles and the rubber will be bonded via the coupling agent. It is considered that by such a chemical bonding, tan .delta. in the vicinity of 60.degree. C. of the particles-incorporated vulcanized rubber composition becomes low, and improvement in the physical properties, such as improvement in the abrasion resistance, is observed.
However, in a case where silica is incorporated as a filler, although it is possible to improve the temperature dependency of tan .delta., there is a problem that the silica filler has a self agglomerating force which is so strong that it is hardly dispersible in rubber and it is poor in processability. Further, there is another problem that the compound cost increases, since it is necessary to incorporate an expensive silane coupling agent usually as much as from 5 to 15% relative to the silica.
Further, if a silane coupling agent having a sulfur atom as mentioned above, is incorporated without incorporating a silica filler to carbon black, there will be a case where tan .delta. in the vicinity of 60.degree. C. of the particles-incorporated rubber composition, will be substantially reduced. However, there will be a drawback that the damping properties tend to be low, since the complex modulus of elasticity (E*) or the dynamic modulus of elasticity (E') is high in a low temperature range (from -10 to 0.degree. C.) under a low strain (from 0.1 to 1%).
On the other hand, it has been proposed to subject carbon black to be incorporated to a rubber composition to surface treatment with silica or a silane. For example, JP-A-53-100190 or JP-A-61-291659 discloses a method of mixing an organic compound or an organometallic compound of silicon dissolved in a solvent or in water, with carbon black, followed by drying. Specifically, as the silicon compound, dimethylpolysiloxane or silicon oil is employed. Further, JP-A-56-38357 proposes hydrophobically surface-treated carbon black having the surface treated with a cyclic alkylpolysiloxane or a silicon compound. JP-A-58-125249 discloses carbon black having the surface coated with a silane coupling agent dissolved in a solvent.
JP-A-63-63755 proposes a method wherein carbon black is dispersed in water, and sodium silicate is neutralized with sulfuric acid, whereby amorphous silica is precipitated on the carbon black surface. However, none of these methods can be said to be suitable for industrial application from the viewpoint of the economy or the process such as removal or recovery of the solvent, or the operation for neutralization. Further, JP-A-4-233976 proposes to chemically modify carbon black with a certain specific organic silicon compound (specifically, a sulfur-containing silane coupling agent having a certain specific structure). However, this method also has a processwise or economical problem, e.g. in the extraction operation or post-treatment for modification. Besides, in these methods for treating carbon black with a surface treating agent such as silica or a silane, it is expected also that the surface treating agent will not sufficiently uniformly attach to the surfaces of all carbon black.